Patterning technologies using charged particle beams such as electron beams and ion beams are expected as the next-generation technology to replace photolithography.
To enhance productivity when charged particle beams are used, it is important to improve the sensitivity of resist. Thus, the mainstream approach in the technical field is to use a highly sensitive chemically amplified resist, in which acid is generated in the portions exposed to light or irradiated by charged particle beams, and then to facilitate crosslinking reactions or decomposition reactions by applying heat known as post-exposure bake (PEB).
Also, as the patterns in semiconductor devices have become more microscopic in recent years, controlling resist patterns on a scale of several nanometers has been required.
When patterns are formed using charged particle beams, especially on an insulative substrate, electric fields generated by electrical charge-up in the substrate may cause the orbit of charged particle beams to be curved, and desired patterns are hard to obtain.
Therefore, to solve such a problem, a technology such as follows is known to be effective: a conductive composition containing a conductive polymer is applied on a resist surface to form conductive coating film so that the resist surface is coated by the conductive film.
As for a method for forming a conductive coating film on the surface of the resist-layer in electron-beam lithography, coating a conductive composition containing a water-soluble conductive polymer and a surfactant on a resist layer (substrate) is known. For example, Patent Literature 1 (JP2002-226721A) discloses a conductive composition containing a water-soluble conductive polymer having a sulfonic acid group and/or a carboxyl group, a water-soluble polymer having a nitrogen-containing functional group and a terminal hydrophobic group with a weight average molecular weight of 1000 to 1500, and a solvent.
Meanwhile, when a contaminant is contained in the above conductive composition, problems such as line disconnection may occur after patterns are formed by using electron beams. Thus, before applying on a resist layer, the conductive composition is put through microfiltration using a filter with a hydrophobic membrane. However, clogging occurs frequently during microfiltration of the conductive composition described in Patent Literature 1, causing problems such as replacing the filter each time it clogs.
Also, when a conductive composition containing a conductive polymer is applied as an antistatic agent in an electron-beam lithographic process of a semiconductor, the coating performance of the conductive composition and its effect on a substrate or the resist coated on the substrate are known to be in a tradeoff relationship.
For example, when additives such as an anionic or a cationic surfactant are added to improve the coating performance of a conductive composition, the acid or base derived from the surfactant adversely affects the resist properties, thus causing problems such as failure of a predetermined pattern to be formed.
In addition, as an antistatic agent applicable to a next-generation process for semiconductor devices, a conductive composition is required to be capable of forming conductive coating film with surface roughness on which even more complicated and fine patterns can be formed, namely, coating film with less surface roughness.
However, in the conductive composition described in Patent Literature 1, it has a problem that cannot use for the resist surface in a next-generation process because the surface of the conductive coating film is too rough.
Also, when the conductor containing the conductive composition of Patent Literature 1 is used for a longer period of time under a temperature of 100° C. or higher, the resist laminate or the like coated on a substrate tends to corrode, causing problems such as a reduction in film thickness when applied on a positive resist, for example.
As described above, conventional technologies have not provided conductive compositions which exhibit excellent coating performance and surface roughness of a conductive coating film, while showing less impact on the resist layer, so as to be applicable to a next-generation process for semiconductor devices.
In addition, as a conductive polymer capable of expressing conductivity without adding a doping agent, self-doped polyanilines having acidic substituents are known. Here, “self-doped” means that a dopant is present in its own structure and is capable of doping without the addition of any doping agent.
To synthesize a self-doped polyaniline having acidic substituents, a method is proposed to polymerize aniline having acidic substituents, for example, aniline with a sulfonic acid group or aniline having a carboxyl group, using an oxidizing agent in the presence of a basic reaction auxiliary.
Conventionally, a polyaniline having acidic substituents is known to be hard to polymerize by itself and thus it has been difficult to produce aniline with a high-molecular-weight. However, using a polymerization method conducted in the presence of a basic reaction auxiliary, a high-molecular-weight polymer can be produced.
Moreover, an acidic-group-substituted polyaniline obtained by the above method exhibits excellent solubility in both acidic and alkaline solutions.
However, when a polyaniline having acidic substituents is polymerized using an oxidizing agent in the presence of a basic reaction auxiliary, a conductive polymer is usually obtained as a reaction mixture that contains residual monomers as well as byproducts generated through side reactions, such as oligomers, acidic substance (sulfate ions or the like, which are decomposed products of monomers or oxidizing agent), basic substance (ammonium ions or the like, which are decomposed products of basic reaction auxiliary or oxidizing agent) and the like. Thus, the degree of purity has not always been high.
In addition, due to its molecular weight and physical properties such as strong base properties, the basic substance cannot steadily neutralize the acidic group of a polyaniline having acidic substituents. Thus, the acidic group site of the polyaniline having acidic substituents tends to be subject to hydrolysis and is unstable. Accordingly, when a polyaniline having acidic substituents is coated on a resist layer and heat is applied on the coated resist layer to form a conductive coating film, the acidic group tends to be easily eliminated.
In the present application, residual monomers and sulfate ions, as well as the acidic group eliminated from a polyaniline having acidic substituents, are collectively referred to as “acidic substances.”
Therefore, when a polyaniline having acidic substituents is applied to a chemically amplified resist, and when exposure-to-light, PEB treatment and development are conducted while the conductive coating film (conductive film) remains on the resist layer, the acidic substances or basic substances tend to migrate to the resist layer. As a result, deformation of patterns, variations in sensitivity or the like occur, adversely affecting the resist layer.
More specifically, if a resist layer is a positive type, when acidic substances migrate from the conductive coating film to the resist layer, the unexposed part of the resist layer is dissolved during development, causing a reduction in film thickness of the resist layer, narrowed patterns, sensitivity change toward a higher sensitivity range and the like.
On the other hand, when base substances migrate from the conductive coating film to the resist layer, the acid component of the exposed part is deactivated, causing change in patterns, sensitivity change toward a lower sensitivity range, and the like.
Also, if a resist layer is a negative type, migration of byproducts from the conductive coating film to the resist layer will cause an opposite result in each of the above.
Accordingly, to stabilize the acidic group of the polyaniline having acidic substituents, Patent Literature 2 (JP2011-219680A), for example, proposes a method for neutralizing the acidic group site by adding a basic compound to a conductive polymer solution from which byproducts or the like have been removed by an ion-exchange method.
According to the method described in Patent Literature 2, by adding a basic compound after byproducts or the like have been removed, the basic compound forms salts with the acidic group of the polyaniline having acidic substituents, thereby preventing elimination of the acidic group. Moreover, a basic compound tends to form salts through reactions with residual monomers or sulfate ions. Thus, migration of acidic substances from the conductive coating film to the resist layer is suppressed.
Furthermore, the patent literature discloses that if quaternary ammonium compounds such as tetramethylammonium hydroxide (TMAH) or tetraethylammonium hydroxide (TEAH) are used as a basic compound, a reduction in the film thickness of a chemically amplified resist is prevented and the heat resistance of the conductive composition is enhanced.
Also, Patent Publication 3 (JP H5-171010A) discloses that a conductive polymer solution is stabilized if a diamine compound such as urea is added as a basic compound. Patent Publication 4 (JP2006-117925A) discloses that adding a divalent or higher aliphatic basic compound to a conductive polymer prevents a reduction in the film thickness of a chemically amplified resist. Moreover, Patent Literature 5 (JP 2010-116441A) discloses that adding an inorganic salt such as sodium hydroxide enhances the heat resistance of the conductive composition.
Further, Patent Literature 6 (PCT Publication WO2012/144608) discloses if 0.3 to 0.5 mol equivalent of tris(hydroxymethyl)aminomethane is added to 1 mol of a unit having an acidic group among the units of a conductive polymer, the heat resistance of the conductive coating film is improved.
However, using methods for adding a basic compound to a conductive polymer solution described in Patent Literatures 2 to 6, migration of acidic substances from the conductive coating film to the resist layer can be suppressed to a certain degree, but such migration needs to be suppressed even further to satisfy the level of performance required as wiring in semiconductor devices has become even finer in recent years. In addition, methods described in Patent Literatures 2 to 6 are not sufficient to suppress elimination of the acidic group from an acidic-group-substituted polyaniline.